A comet is an enormous ball of ice, and a star is a much more enormous ball of fire. Normally, bringing the two together leads to a spectacular if predictable tail of melting liquid, the signature flare of a comet as it orbits a star. That tail grows and shrinks as the comet moves toward and away from its star — but what if a comet were to move too close to the sun? We may be about to see the answer first hand, as this Thanksgiving Day a “sungrazing” comet dubbed Ison will reach the point in its orbit closest to our star.

Astronomers aren’t sure if Ison, which is judged to be about 5 kilometers across, will shatter under the extreme stress or evaporate and survive to put on an incredible show. If the comet survives its close graze to the sun, it will lose a good portion of its mass. It may in fact be evaporated entirely — a spectacular prospect that would should be visible to the naked eye.

In approaching the sun, Comet Ison is picking up significant speed. Clocked in January at 64,000 kilometers per hour, today it has almost quadrupled that speed to a whopping 240,000 kph. At perihelion it will be closest to the sun, and reach its greatest orbital speed. Ison will also be one of the most luminous comets in some time, and it is expected to briefly outshine Venus in the night sky.

The Oort Cloud

Interestingly, astronomers differ on Ison’s origins. Initial estimates said that the comet was approaching the sun on a hyperbolic trajectory, meaning that it is a newcomer to our system, just now approaching the sun after leaving the Oort cloud. New calculations, however, show that Ison could be moving along an eliptical orbit, and that it has probably passed close to the sun before. In either case, this is an exceedingly rare event, either the death of a veteran comet that flew one too many missions, or of a newcomer that could never take the heat in the first place.